54 CLINICAL BACTERIOLOGY. 



of intoxication especially predominate over those due to the 

 local effects of the bacterial irritants. 



We have thus far considered two modes of immunization 

 (i) the attenuation-method (Pasteur), in which attenuated 

 cultures of living bacteria or attenuated toxins are employed ; 

 and (2) the dilution-method, in which minimal amounts of 

 virulent cultures or toxic metabolic products are employed. 



Immunity to certain microorganisms may further be con- 

 ferred by introducing them into the organism to be vacci- 

 nated through a portal of entry that is different from that 

 through which the same bacteria gain entrance under 

 natural conditions. On introducing the virus of pleuro- 

 pneumonia of cattle into the extremity of the tail, there 

 results, for instance, insignificant local disease, which is 

 followed by immunity. The subcutaneous injection of 

 cholera-cultures for the purpose of conferring immunity to 

 Asiatic cholera may also be mentioned. 



More difficult of comprehension is a smaller number of 

 methods of immunization, in which, apparently, the bacteria 

 themselves take no part. Thus, for instance, injections of 

 hydrogen dioxid are said to afford protection against sub- 

 sequent infection with diphtheria-bacilli. -Under these cir- 

 cumstances it is to be assumed that either the previously 

 injected material remains at the seat of injection, where it 

 encounters the bacilli subsequently introduced, or their 

 poisons, and these are, to a certain degree, thereby at- 

 tenuated within the body ; or there may result an intensifi- 

 cation of those forces of the organism that oppose barriers 

 to the infection forces that, as has already been said, are 

 present also in the most susceptible organism, at least in a 

 rudimentary degree. But few immunizing methods of this 

 kind are known, and they are of little significance as com- 

 pared with the large number of methods already mentioned, 

 in which the bacteria themselves, or their metabolic pro- 

 ducts, participate in the establishment of immunity. 



The last method of immunization to be considered is 

 that suggested by Richet and Hericourt, and devised by 

 Behring and Kitasato namely, immunization by means of the 

 blood-serum of immunized animals. Though demonstrated 

 by the investigators named as useful only for staphylo- 

 coccous septicemia, as well as for diphtheria and for tetanus, 

 this method was subsequently employed against the infec- 

 tions due to other bacteria, and found to be practicable. 



